Known engine camshafts comprise a shaft, cam lobes for valve actuation and journal bearings all formed in a single casting. After casting a blank having the approximate desired dimensions, the bearings and cam lobe surfaces are machined and/or ground to the precise required dimensions.
In an engine where the camshaft is mounted overhead, that is over the valve mechanism, the journal bearings can have a diameter smaller than the cam lobes because it is possible to assemble pillow blocks about the journal bearings. On the other hand, in a push rod engine where the camshaft is mounted in the side of the engine block and actuates the valve mechanism through cam lobe followers and pushrods, the journal bearings must have a diameter at least as large as the cam lobes to allow the camshaft to be inserted from one end into a longitudinal camshaft bore in the engine block.
The weak point in either type of camshaft lies between the journal bearing at the driven end of the camshaft and the adjacent cam lobe as a result of the torsional bending forces exerted by the drive arrangement on the camshaft. This is because some engines have an arrangement where one or more power consuming accessories are driven through the camshaft drive gear which meshes with a crankshaft drive gear. These accessories, which can consume up to 100 hp., are driven from the camshaft drive gear. The reaction forces between the accessory gear and the camshaft drive gear impose a significant bending load on the camshaft. This bending load is resisted by the camshaft bearing nearest the drive gear but is passed to the camshaft between this bearing and the first cam lobe.
Conventionally, the nose of this first cam lobe (i.e. its axial end) is formed by first casting the region of the camshaft between the cam lobe and the journal bearing to be of circular cross section having a diameter just under the diameter of the baseline circle for the cam lobe. For increased strength, however, this cam nose region is then machined down to a slightly smaller and smooth circular cross section. The reason for this last step is that a cast region is more prone to cracking and even though the machining reduces its cross sectional area it increases its strength.
Such a method of manufacture results in a cam nose of moderate strength and to achieve the desired durability it has been necessary to use material of a high strength, such as steel, thereby adding considerably to the cost of the camshaft.